Power amplifiers are used in transmitters to rebroadcast, at high power levels, amplitude modulation (AM) signals, frequency modulation (FM) signals, and quadrature amplitude modulation (QAM) signals. As is known, information is carried in the amplitudes of AM and QAM signals. Because of this, transmitters must accurately rebroadcast these signals, thus the power amplifiers within the transmitters must have high fidelity, i.e. linearly rebroadcast the signals they receive.
A typical linear transmitter includes a baseband signal generator that produces a baseband signal output in response to a digital input. The digital input is mapped to a constellation corresponding to output voltage level and phase. An example of such a modulation is QAM. The baseband signal is fed to an up mixer to create a radio frequency signal, which is then filtered and fed to a power amplifier for transmission over the air.
To maintain linear performance, the power elements (usually transistors) are operated at levels much below their rated output power levels. Thus, to achieve high output power levels, many power amplifiers include cascaded elements, such that the amplified output of one element is the input of the next element. In this manner, a small change in a first stage typically produces a large change in the final stage output. For this reason, the power amplifier utilizes a feedback control loop to maintain output power levels.
In some applications increased linearity was desired, and Cartesian feedback was found to significantly enhance linearity. Cartesian feedback comprises sampling the output of the power amplifier, down mixing it at the same frequency the up mixer uses (but with a phase shift which is pre-aligned to insure negative feedback), then feeding the down mixed signal back to a combiner or summing node and subtracting it from the output of the baseband signal generator. Instead of feeding the baseband signal directly to the up mixer, the difference of the baseband signal and the down mixed signal is amplified and fed to the up mixer. If the gain of the power amplifier goes up, the increase in gain is automatically compensated by the Cartesian feedback loop which reduces the level of the signal fed to the up mixer.
In practice, a power amplifier stage may use a power field effect transistor (FET) such as a lateral diffused metal-oxide-semiconductor (LDMOS) transistor. The gate of the transistor is coupled to an RF input waveform at a given fundamental frequency, which typically comprises an RF carrier modulated with information to be communicated over the air. The transistor then generates an amplified RF signal at its output, which creates a voltage response between the drain and source, which is a function of a drain current generated in the transistor and an impedance, which is based on an output matching network. Generally, the drain current comprises a spectral component at the fundamental frequency and spectral components at one or more harmonic frequencies. In a Class AB amplifier, these harmonic components are primarily even order components produced by the near cutoff DC gate bias, which only allows drain current to flow during the positive half cycles of the gate voltage waveform
In one application, the transistor is operated near its maximum operating frequency. In this case, it is usually sufficient to design the matching network to correspond to or “dominate” only the fundamental component of drain current. Dominate herein means to specifically load a given spectral component of the output current of a power amplifier device with a desired impedance to create a corresponding desired spectral component of the voltage response at the output of the power amplifier device. In this application, the device output capacitance presents a low impedance to the harmonic components of the drain current, so that they do not substantially increase the drain voltage over that which is produced by the fundamental frequency drain current feeding into the fundamental frequency load impedance.
In another application, which is of primary interest in this invention, the transistor is operated at much less than its maximum operating frequency. In this situation, a high harmonic load impedance can be created when the RF power device output capacitance resonates with the residual inductance presented by the output impedance matching network at one or more of these harmonic frequencies. A high harmonic drain voltage component will be generated if there is a substantial drain current present at one of these “high impedance” harmonics. This is a particular problem at the even harmonics, where the Class AB bias tends to generate strong even harmonic currents, as noted above. These harmonic voltage components will add to the fundamental component of the drain voltage to produce a peak drain voltage which can exceed the device's drain-source breakdown voltage (BVds). When this happens, the high avalanche current produced by drain-source junction breakdown can destroy (burnout) the RF power device.
There are known techniques, such as those described in U.S. patent application Ser. No. 11/345,573 which can control critical harmonic impedances well enough to permit a moderate bandwidth VHF amplifier to operate without device failure if the input drive level is no more than 3 dB above the drive level required to produce nominal output power. This drive level limitation is common in constant envelope (FM) power amplifiers, where linearity is not required.
Linear power amplifiers, on the other hand, typically operate all stages, except the final stage, at levels that are 10 dB (or more) below their maximum output power capability. This is done to minimize the buildup of distortion in these “driver” stages. While this excess drive capability does not create problems in normal operation, it does mean that any stage, including the final stage, could see 10 dB or more overdrive in a “runaway” condition. Experience has shown that the techniques described in U.S. patent application Ser. No. 11/345,573 may be inadequate to prevent RF power device burnout in the event of such a severe overdrive.
One such “runaway” condition can arise if the feedback phase shift in a Cartesian feedback linearizer is set incorrectly, which can be caused by a hardware or software failure during the training process used to pre-align this phase shift. In this case, the Cartesian feedback linearizer can become a chaotic oscillator whose output power peaks will drive the PA driver chain to its maximum output power capability. In this case, burnout can occur when this overdrive causes the instantaneous drain voltage of an RF power transistor to exceed its breakdown voltage.
Currently, there exists no suitable method for the application designer to address this problem. In general, there are many forms of power amplifier protection circuits available, but none are known which can directly sense instantaneous drain voltage in a MOSFET power amplifier and can respond quickly enough to prevent burnout due to device breakdown.
One attempt at a solution is to use harmonic termination circuits to reduce peak drain voltage in a power amplifier stage at normal power levels. Although an improvement in the art, such solutions have demonstrated that harmonic termination circuits alone are not sufficient to prevent drain voltages from exceeding breakdown in extreme overdrive situations.
Another improvement in the art, U.S. Pat. No. 5,426,395, describes a circuit which protects a power amplifier from damage due to excessive operating power levels by increasing the level of negative feedback used in a feedback based power amplifier linearization system. This approach has two problems. First, it relies on sensing power amplifier output power after impedance matching, which does not directly correspond to peak drain voltage. Second, increasing feedback will actually increase power amplifier overdrive in cases where the feedback phase is positive due to a misaligned Cartesian feedback linearizer.
Yet another solution is U.S. Pat. No. 6,580,318, which is similar to U.S. Pat. No. 5,426,395, but uses a power control feedback loop to reduce power amplifier drive and/or gain when output power exceeds a preset threshold. In addition to the sensing problem in U.S. Pat. No. 5,426,395, this circuit has a slow response time due to the need limit power control feedback loop bandwidth in order to maintain loop stability.
Yet another solution is U.S. Pat. No. 6,850,119, which discloses a circuit to prevent HBT power amplifier damage by reducing power amplifier drive and/or RF device bias levels. However, this reference uses a different overload detection mechanism, i.e. sensing changes in the base bias of an HBT device. This will not work for MOSFET devices, whose gate bias will not change as a function of output or input power (or any other condition), until the device is actually destroyed. In addition, the disclosed protection circuit contains linear feedback, so its response time is constrained in the same manner as in U.S. Pat. No. 6,580,318. In addition, both U.S. Pat. Nos. 6,580,318 and 6,850,119 contain linear feedback loops, which must have limited bandwidth, and hence long response times, due to the need to maintain loop stability.
Therefore, a need exists for a method and apparatus that provides protection of power amplifiers from excessive operating power levels. It would also be an advantage to provide protection at a faster response than prior art protection circuits.